Alternating current (AC) signals are commonly transmitted between a signal source and a load via a transmission line having two or more conductors. Transmission lines provide an inexpensive means of carrying relatively high currents across distances with relatively low loss.
A transmission line has a number of different failure modes which can prevent the signal produced by the source from reaching the load and/or can damage the source. For example, suppose a two-conductor transmission line is used to connect a source of alternating current to a load. If one or both conductors becomes disconnected from the source or the load, no signal produced by the source reaches the load. This type of fault is usually relatively easy to diagnose, since it occurs at a point in the transmission system where faults are expected and where the transmission line is easily accessible (e.g., at a connector connecting the source to the transmission line or at a connector connecting the transmission line to the load).
It is far more difficult to diagnose problems compromising the integrity of the transmission line itself (e.g., short-circuits between the conductors of the transmission line, and discontinuity or breakage of one or more conductors). These types of faults are often diagnosed by visually inspecting the transmission line, a time-consuming and often unreliable method of fault detection. Hence, transmission lines are usually simply replaced if the transmission line terminations seem to be fault-free and the transmission system nevertheless does not work properly.
Additionally, it is often difficult to determine whether a transmission line has failed and, if it has, whether the failure might result in damage to the signal source. A short-circuited transmission line may draw excessive current from the signal source and damage the source. Even signal sources having output protection (e.g., current limiting or the like) typically simply shut down without indicating to a user that the transmission line has failed or how it has failed.
DC resistance measurements can sometimes be used to determine how a transmission line has failed. Unfortunately, such measurements generally cannot be performed when a signal is present on the transmission line. Disconnection of the transmission line from the signal source or the load interrupts transmission service and increases system down time. Intermittent faults present particular difficulties, since resistance measurements cannot be performed continuously while the transmission line is carrying a signal, and yet, taking the line out of service to test it may not demonstrate a fault exists. Moreover, failure of the transmission line in a manner which changes its AC impedance but not its DC resistance cannot be detected by DC resistance measurements and yet can degrade transmission line performance. Resistance measurements are therefore not an entirely accurate method of detecting all faults in AC systems.
The prior-issued United States patents listed below disclose representative fault and signal detection circuits which may be relevant to the present invention:
______________________________________ Chaudhary 4,253,056 1981 Shirey et al 4,488,110 1984 Brandt 3,838,339 1974 Vitins 4,179,651 1979 Dudley 4,334,188 1982 Ebert, Jr. 4,208,627 1980 Knauer 3,982,158 1976 Kimzey 3,911,360 1975. ______________________________________
Some of the patents listed above disclose DC voltage detectors which can be continuously connected to the DC voltage to be monitored. For example, the Chaudhary patent discloses a detector circuit including a voltage comparator, voltage dividers and diodes. Chaudhary's circuit can be continuously coupled to a DC power supply output to detect an unwanted ground coupled to either terminal of the power supply. Shirey et al discloses a DC voltage monitoring circuit which illuminates a green LED when the monitored DC voltage exceeds a predetermined reference voltage, and illuminates a red LED when the DC voltage is less than the reference voltage.
The Brandt patent teaches a digital signal test circuit for detecting low, high and open voltage levels, as well as current levels indicative of an open-circuit condition. Indicators associated with each of the detectors include light emitting diodes (LEDS).
Vitins shows a system for detecting short circuits in an electrical line by utilizing both current and voltage signals. The current and voltage signals are weighted relative to one another, an amplitude comparision is ultimately carried out, and short circuits are also detected.
Dudley shows (in FIG. 8b) circuitry including a light emitting diode which can detect open circuits. Ebert, Jr. teaches a simple circuit for detecting shorting of a periodic signal source. The Knauer patent teaches a power distribution control system which compares actual line current to simulated fault current when the line current exceeds a predetermined threshold level.
Kimzey teaches a voltage monitoring system using a flip-flop and an LED which provide an indication that the monitored voltage has fallen to below a threshold and maintain the indication of failure after the voltage has returned to its normal level.
A further reference, The Radio Amateur's Handbook (American Radio Relay League, 54th ed.), discloses a transmatch circuit at page 584 which includes a standing wave ratio measuring circuit inductively coupled to an RF transmission line. The standing wave ratio measuring circuit provides an analog indication (on an ammeter) of the amplitude of current flowing through the transmission line.
Although complex fault detectors have their purposes, a simple and inexpensive AC transmission line fault detector which can be continuously coupled to a transmission line without degrading AC signal transmission and which can indicate when the transmission line has failed and how it has failed would be highly desirable.